This invention relates to a small size printing head which uses wire elements to print by a plurality of dots.
Wire dot printing heads are widely used in recent printers. FIG. 1 illustrates a conventional wire dot printing head with the lower half in section. The wire dot printing head comprises a casing 1 having a circular section. A plurality of electromagnets 2 and armatures 3 operated by the electromagnets 2 are arranged around the central axis of the casing 1. The armatures 3 are connected at their outer end portions for pivotal movement relative the respective electromagnet 2. In a cylindrical portion 1a intergal with the casing 1, a pair of guide plates 5a and 5b made of a plastic material are provided for slidably guiding wires 4 to be used for wire dot printing. At an end of the cylindrical portion 1a is provided a highly wear-resistant guide plate 6 that supports the outer ends of the wires 4. The other ends of the wires 4 are formed into contacting portions 7 connected with the armatures 3. A coil spring 8 is provided around the contacting portion 7 so as to urge the same in a direction to separate the armature 3 from the electromagnet 2.
Upon energization of one or more of the electromagnets 2, the armature 3 corresponding to the energized magnet 2 is attracted by the magnet 2 against the force of the coil spring 8 to cause the outer ends of the wires 4 coupled with the armatures 3 to project outwardly from the end of the cylindrical portion 1a of the casing 1 so as to impact on a recording paper through an inked ribbon. By combining the movement of a carriage that supports the printing head with the projecting movements of the wires 4, a required dot printing operation can be carried out on the recording paper.
In the above described conventional printing head, however, the armatures 3 and the coil springs 8 are directly coupled with the inner ends of the wires 4, while the outer ends of the wires 4 are guided by the guide plate 6 in a minutely spaced apart relation. Thus the wires 4 inevitably follow a curved path from the positions surrounded by the coil springs 8 to the positions supported by the guide plates 5a and 5b. As a consequence, frictional forces are created between the guide plates 6, 5a and 5b and the wires 4 when the wires are returned to their original positions by the resilient forces of the springs 8, thus increasing the sliding resistances of the wires 4. The frictional forces created between the wires 4 and the guide plates 6, 5a and 5b not only adversely affect the operational life of the printing head, but also necessitate coil springs 8 of large spring forces and electromagnets 2 of large capacities be provided, thus increasing the power consumption.
Furthermore, each of the wires 4 has a diameter ranging from 0.2 to 0.35 mm, and must be passed through the guide plates 5a and 5b and the guide plate 6 so as to be extended along a bent passage, and each of the coil springs 8 must be placed around the stem portion of the wire 4, and therefore the assembling work of the conventional printing head is extremely troublesome. Since the required lengths of the wires 4 are different by the positions of the wire ends, it has been an ordinary practice that the outer ends of the wires 4 are cut away after the insertion of the wires 4 through the aforementioned passages so that the wire ends are arranged in a plane. This requires a difficult production step requiring a high precision and a long operation time. As a consequence, there is a limitation in reducing the cost of the printing head, which constitutes a difficulty in reducing the cost of small size printers for personal computers and the like which are required to be of low cost.
In addition, since the wires 4 in the conventional printing head are extended along bent passages, the cylindrical portion 1a of the casing 1 is required for straightening the movement of the end portions of the wires. The portion 1a increases the space of the entire printing head and deteriorates the space factor of the printing head.